This invention relates to a process for preparing a polymer emulsion containing colloidally suspended therein an interpenetrating polymer network wherein a first polymer network is intertwined on a molecular scale with a second polymer network and optionally additional polymer networks. The polymer emulsion of this invention is useful as a binder for paper coatings and printing inks.
In the preparation of a coated cellulosic web, e.g. a paper web, there is used a pigment, such as clay or the like, sometimes with other materials such as, for example, a soluble pyrophosphate which may act to disperse the pigment in water and stabilize the pigment in water. This mixture, commonly termed a pigment "slip" or, since it usually contains clay, a clay "slip", is then compounded with a binder or adhesive material to produce a composition known in the art as a coating "color", which is useful for coating a cellulose web, e.g. a paper or paperboard web. Substantial quantities of the binder are used, and, accordingly, the composition and characteristics of the binder are of great importance in determining the qualities of the finished coated web. It is important that the binder contributes to the coating or the finished coated web a high degree of brightness, smoothness and gloss, and a good finish and feel after calendering. In addition to these basic qualities required in coatings, the coating color must flow smoothly and evenly so that it can be applied to the cellulosic web at sufficiently high speeds to be economical in ordinary coating processes; and the coating must have high strength, to permit subsequent printing on the coated paper without "picking," i.e. it must have good "pick" characteristics.
Polymer emulsions are useful as a coating binder for paper and paperboard. Paper is coated to provide a smoother surface with increased strength, whiteness and absorbability in order to provide a better surface on which to print. Coating formulations for paper and paperboard can contain a variety of binders including all-latex binders, protein-latex binders, all-starch binders or latex-starch blends. The end use of the paper and, in particular, the method by which it will be printed, may determine which binder type is used in the coating. The major printing method is the offset method in which both water (fountain solution) and an oil based ink are applied to the paper coating. The rate of absorption of the water layer and the ink into the coating is critical to producing a desirable high quality printing.
Styrene-butadiene copolymers are commonly used latex binders, followed by polyvinylacetate, vinylacetate-acrylic copolymers, ethylene-vinylacetate copolymers and all acrylic polymer emulsions. Styrene-butadiene and vinylacetate binders are widely used because of their low cost. The major drawback of styrene-butadiene binders is the poor water absorption giving high SIWA brightness values. High SIWA (simultaneous ink and water absorption test) brightness values mean the coating did not absorb the initially applied water layer and the subsequent ink application failed to penetrate this layer and absorb into the coating. The incomplete ink coverage produces a weak or spotty image. Vinyl acetate binders are often too water absorbent, resulting in press roll fountain solution milking. This problem is the converse of the high SIWA brightness problem. Fountain solution milking occurs when the coating absorbs so much water (fountain solution), that the coating becomes solubilized in the fountain solution and the binder and clay so dissolved give the solution a "milky" appearance. This condition can be predicted by the Adams Wet Rub Test.
Printing inks will generally contain a pigment or dyestuff and a vehicle as well as supplemental ingredients to impart special characteristics to inks such as driers, waxes, lubricants, reducing oils, antioxidants, gums, starches and surface active agents. The function of the vehicle is to act as a carrier for the pigment and as a binder to affix the pigment to the printed surface. The vehicle can contain in various combinations, resins, oils and solvents depending upon the printing method. For example a flexographic vehicle can contain either alcohols, water, or other fast evaporating solvents with suitable resins and gums, while a gravure vehicle can contain low boiling hydrocarbon solvents with gums and resins.
Printing inks use natural or synthetic resins to impart the properties of hardness, gloss, adhesion and flexibility which is important in the formulation of binders for the pigments. Synthetic resins are prepared by polymerization involving condensation or addition reactions between relatively small molecules. Various synthetic resins are utilized in different ink applications. For example, pure phenolic resins are used in conjunction with tung oil as a varnish for letterpress and lithographic inks; rosin modified phenolic resins have widespread use in all types of ink vehicle systems; alkyd resins are used in paste inks; polystyrene resins and copolymers thereof (e.g. with maleic anhydride) can be made water soluble for use in water based inks; thermoplastic polyamides are used in liquid inks; and acrylic and methacrylic polymers and copolymers thereof are used in flexographic, photogravure and tinprinting inks and high gloss lacquers. The resins are often used in combination with other resins or film forms to impart the properties desired for a particular application.
The polymer emulsion of this invention provides a binder which is suitable for use in a coating, especially as a coating for paper and paperboard, and in a printing ink.